Nest usurpation is an 'edge effect' for Carolina chickadees Poecile carolinensis

During 1995–1997, we monitored Carolina chickadee Poecile carolinensis nests in a fragmented forest landscape in northcentral Ohio, USA. Nest success was positively correlated with woodlot area and most nest loss was due to nest destruction by house wrens Troglodytes aedon . During 1998 and 1999, we conducted an experiment in both large (>6.8 ha) and small (<6.8 ha) woodlots in which we gave chickadees a choice of nesting on edges of woodlots (preferred wren habitat) or in the center of woodlots. We found no difference in nest success between large and small woodlots, but regardless of woodlot size, nest success was lower on edges than in the center. In the experiment, 100% of nest loss was due to nest destruction by house wrens. Given a choice, Carolina chickadees preferred to nest centrally more often than on edges. These results suggest that in fragmented landscapes where house wrens are common, nest destruction by house wrens is a major cause of nest failure in the Carolina chickadee. Such edge-dependent interspecific nest-site usurpation has not been previously recognized as a potentially important selective factor in nest site selection.